The student-designed car might not be practical, but it is efficient.
Don't expect to see these in dealerships anytime soon, but students at Utah's Brigham Young University have built a car which can achieve 1,300 mpg, and possibly even more than that. The car was built in order to participate in the SAE Super Mileage Competition, a contest for students to build hyper-efficient vehicles. The cars are ran on a track with just 20 grams of fuel in order to determine their mpg capability.
BYU took second last year with a 1,135 mpg car. This year, the students have aimed at 2,000 mpg, but expect at least 1,300 mpg. The new car weighs just 99 lbs, a good 22 lbs less than last year's. There are some improvements in the engine insulation as well, but the engine itself is quite humble. The lawnmower-sourced power plant will get the car up to the SAE's 15 mph cruising speed and then shut itself off when it isn't needed. Lightness alone is the key here, with nothing else fancy needed for high mpg numbers.